Video signal generator



Aug. 9, 1966 P. ESSINGER. mLV 3255,12

i I VIDEO SIGNAL GENERATOR Filed Dec: 51, i963 2 Sheng-Sheet 2:

limi-tations of priorsuch generators.

` r Other objects and advantages pear as the detailed description proceeds in the light of the 4drawings formi-ng apart of this application and in` which:

Patented August 9, 1966 3,255,812 VIDEO SIGNAL GENERATOR Pierre Esainger, Yorktown Heights, Evon C, 'Gr-ennuis, Chappaqua, and Philip F. Meagher, Mount Kisco, N.Y., assignors to International BusinessMachines Corporation, New York, N.Y., a corporation of New York'` Filed Dec. 31, 1963, Ser. 'No.,334,824

9 (Zlsnirns.4 (Cl. 178--7.2)

The present invention'relates to video signal generators and, more particularly, to arrangements for generating a video signal by the use of a moving spot of light which traces a scan raster to explore incremental-area brightness vcontrasts of an image to 'be transmitted. The invention has particular utility for video signal generation by use of so-called dying 'spot scanners wherein theelectron beam luminous spot on thefuorescent screen of thetube and is l deflected in two directions at'different velocities to create parallel scanning lines suitablefor exploratory illumina- -tion of an image area.

Flying spot scanners are widelyuse'd as scanners of moving picture film in television video signal generation. They have also `found utility in character recognition applications wherein the brightly luminoustrace Paltem of r the cathode ray tube is imaged onto the surface of a printed or handwritten document. Light reflected from the document, and modulated in intensity lby the contrast between the printed or handwritten characters and the background surface upon which they appear, is received by a photo-multiplier tube to generate aform of.video signal which may then be used to recognize individual ones of successively printed or written characters. Representative arrangements of this nature are disclosed in the Meagher et al. application Serial No. 305,254 filed August 29, 1963 and the Greanias application Serial No. 248,585, filed De'- cember 31, 1962, both assigned to the same assignee as the present application. In document scanning applications, exaggerated increase of contrast is desirabley whereas softening or reduction of contrast is undesirable and variations from relatively uniform contrast may often prove troublesome in attaining high accuracy of eharacterecognition;` When video signals as produc'ed, by a dying spot scanner source-of document illumination and associated photo-multiplier tube, it has been foundthat the level of the video signal is undesirably subject to a number of prevailing operational conditions. Amongvthese is the photo-multiplier tube multiplication ratio change by reason of tube drift and its sensitivity to variations of its elec# trical energization, variation lof light, collection efficiency n on different parts ofthe document, non-uniform aging of areas of the fluorescent phosphor screen of the cathode ray tube, and variations of scanning spot lumino'usji'ntensity ,dueI to variations of electron gunelectrical"energization vand incremental-area phosphor luminous eiciency which creates undesirable phosphor noise i-n the video signal.

It isan object of the present invention toy provide a new and improved video signal' generator particularly suitable for document character recognition applications, and one which avoids one or more of the disadvantages and t It is a further object of the invention to provide a novel video signal generator which incorporates compensation for undesirable variations of limage-exploratory scanning light intensity and includes compensation for high-frequency variations occasioned'by phosphor noise prevalent in iiying spot scanners. t

of the invention will ap- FIG. 1 represents in block diagram form the electrical arrangement of a video A*signalgenerator embodyingl the `of a cathode ray tube isfocusedto a smallhigh-intensity y present invention in `a particular formsuitable for document character recognition applications and using a flying spot scanner; and

VFIG.2 shows schematically the electrical circuit arrangement of a controlled power supply systemused in the FIG. 1 generator for energizing a photom-ultiplier tube employed therein. y

Referring now more particularly to FIG. l, the video signal` generator is shown as employing a conventional form of flying spot scanner which includes a cathode ray tube 10 having a cathode electrode 11, a beam-intensity control electrode 12, and first and second anodes 13 and 14 all energized by a power supply system 15. The electron beam ofthe tube 10 is focused by a conventional `magnetic, field focusing structure, not shown, to a small tal line trace and moves at uniform but slower velocity` vertically to trace successive ones of parallel scanning lines uniformly spaced from top to bottom of the fluorescent screen. The image of the moving spot is at all times projected by a lens system 20 and mirror 2l onto a document 22 to explore the incremental area brightness contrasts of printed or handwritten characters appearing thereon.

Light reflected from the surface of the document 22 is vmodulated in intensity by the scanning spot exploration of the image appearing on the document, and this modulated reilectedlight is received by a photo-multiplier tube 25 which operates vin conventional manner to create and amplify an electrical video signal. This video signal is applied from an output circuit of the tube 25 to the input circuit of a conventional direct-current amplifier 26, indicated Vas having substantial direct-current degenerative feedback-through a resistor 27 coupling its output circuit to its input circuit. Such amplifiers conventionally have essentially constant input-circuit to output-circuit gain from zero frequency (direct current) to about 2,000 cycles per second with a higher frequency roll o rate of about 20 db per decade to a zero db frequency of about 2x108 cycles per second. They typically exhibit an input circuit impedance greater than about 104 ohms, an output circuit voltage of L50 volts at an ouput current of $.50 milliamperes.

The video signal amplified by the amplifier 26 is peakrectified by a detectorZS, and the direct current and low frequency alternating current components are supplied to the input circuit of a direct current amplifier 29 similar to the amplifier 26. The amplified video signal in the output circuit of the amplifier 26 is also supplied to a peak detector 30, and the midrange frequency components of the video signal are applied to-a midrange amplifier 3l having substantially uniform gain from'about 10 cycles per second to 5,000 cycles per second. The amplified signals of both of the amplifiers 29 and 31 are supplied to a power supply system 32 to control, in a manner to be explained more fully hereinafter, the magnitude of the 4energizing potentials developed by the power supply system and supplied to energize the numerous electrodcs'of the photo-multiplier tube 25. The amplied midrange frequency components of the video signal may also be supplied,`if desired, from the amplifier 31 through the closed contacts'of a switch 33 anda coupling condenser 34`to 'the cathode electrode 11 of the cathode ray tube 10 for modulation of the cathode ray beam of the latter.

A photo-multiplier tube 38 is positioned to view the illuminated screen 16 of the cathode ray tube 1t) and to generate in an output circuit of the tube 38 an electrical signal having an instantaneous amplitude varying with the instantaneous luminous intensity of the luminous scanning spot as it traces the raster of parallel scanning lines on the fluorescent screen f6. 'l'his electrical signal is supplied to lthc input circuit of a conventional direct current amplifier 39 similar to the amplifiers 26 and 29. 'l'hc midrange frequency components of the signal developed in the output circuit of the amplifier 39 are amplified by a midrange amplifier 41, having the same construction as the midrange amplified 3l. The amplified midrange signal components developed in the output circuit of the amplifier 41 and the amplified signal developed in the output circuit of the amplifier 39 are supplied to individual control circuits of a power supply system 42 to control the magnitudes of the voltages developed by the supply system 42 and supplied to energize the numerous electrodes of the photo-multiplier tube 38.

The amplified signal appearing in the output circuit of the amplifier 39 contains certain high frequency phosphor noise components presently to be considered more fully, and these high frequency components are translated through a high frequency amplifier 45 to the input circuit of a direct current summing amplifier 46. The amplified video signal appearing in the output circuit of the amplifier .26 is also applied to the input circuit of the amplifier 46 but with opposite polarity to the high frequency phosphor noise components supplied to the latter from the amplifier 45. An amplified video signal, relatively free of high frequency phosphor noise" components, is developed in the output circuit 47 of the amplifier 46 for translation to utilizing equipment not shown.

In considering the operation of the video signal gcnerator just described, it may be pointed out at the outset that the video signal developed in the output circuit ofthe photo-multiplier tube 2S by operation of the flying spot scanner source of illumination of the document 22 causes several forms of undesirable spurious amplitude variations to appear in the video signal. Certain of these are occasioned by changes in'the vmultiplication ratio of the photo-multiplier tube 25 due to well -known drift of the tube operational characteristics and its sensitivity to variations of its electrical energization from the power supply system 32. A further undesirable spurious amplitude variation of the video signal is occasioned by variation of the efiiciency with which refiected light is collected by the photo-multiplier tube 25 from different areas of the document Z2. Further undesirable spurious amplitude variations of the video signal are occasioned by non-uniform aging of areas of the fluorescent screen 16 and to nonunitorm area response of the fluorescent screen to the electron beam of the cathode ray tube 10, to variations of the scanning spot luminous intensity due to variations of the l electrical energization ofthe cathode ray tube l beaniforming electrodes, and to incremental-area phosphor luminous ef'liciency occasioned by the particulate structure of the fluorescent screen i6 and which results in undesirable phosphor noise frequency components.

Aside from the phosphor noise amplitude variations of the video signal, the other undesirable amplitude variations of the video signal are either long-term variations of those'occurring at relatively low frequency or are variations that occur at midrange frequencies. For example, undesirable amplitude variations of the video signal resulting from variation of light collection efficiency by the photo-multiplier tube may occur at a midrange frequency if the light collection efficiency varies along a horizontal line scanning movement of the scanning light spot or may occur at a relatively low frequency if the collection efficiency varies in the vertical direction of light spot scanning of the document.

The low frequency and midrange frequency undesirable amplitude variations which tend to appear in the video signal generated by the photo-multiplier tube 25 are am plified by the amplifier 26, are peak detected by the dctcctors 28 and 30, and are amplified by a respective one of the amplifiers 29 and 3l. The amplified signals of the latter amplifiers so control thc power supply system 32 as to vary the energization of the photo-multiplier tu'oc 25 to such extent and in such sense as substantially to reduce the low frequency and midrange frequency undesirable amplitude variations of the video signal. In this` thc detectors 28 and 30 sense any amplitude variations of the video signal occurring from' Zero (direct current) frcquency to an upper frequency limit of the midrange frcquency band translated by thc amplicr 3l. lach of the detectors 28 and 30 includt.` capacitive and resistive conn ponents as will presently be explained more fully, and these components are selected to provide :t sufiicicntly long peak detector time constant that each detector is relatively insensitive to tbe relatively wide band of higher frequency amplitude modulation components which carry the video information of the video .signalA rThus the amplitude control exercised by the units 26 and 28h32 over the video signal developed in thc output circuit ot' the photo-multiplier tube 215 affects compensation only of undesirable video signal peak amplitude variations without affecting the video information carried by the signal.

The units 26 and 26?32 will be recognized as comprising a feedback loop which operates to receive the video output signal of the photo-multiplier tube 25 and, by feedback energirution control over the latter. maintain the amplitude of thc vidco signal substantially constant with respect those amplitude variations which fall within the range of relatively low frequencies to which the detectors 28 and 3() arc responsive. This amplitude control is cflective `to compensate not only for any operational condition which otherwise changes the multiplication ratio (gain) of the photo-multiplier tube 25 itself, but also for those which cfl'cct changes in the luminous intensity of the scanning spot produced by the cathode ray tube iti or which are occasioned by changes of the efficiency with which reflected light is collected by the photo-multiplicr tube 25. Any such operational condition which tends to change the luminous intensity of the scanning spot, such as changes of energization of the cathode ray tube 1t) by the power supply 15 or non-uniform aging of arcas of thc fluorescent screen f6 of the tube tti, can be further compensated by closing thc switch 33 to supply to the cathode lll of the tube l0 the midrange-frequency video signal components from the amplihcr 3l. This video signal energization of the cathode il modulates the cathode ray beam to such extent and in such sense as to maintain substantially constant the luminous intensity of thc scanning spot with respect luminous intensity variations occurring at frequencies within the midrange band translated by the amplifier 3l. lf non-uniform aging of areas of the fluorescent -scrccn liti is a contrilmtory factor to such change of scanning spot luminous intensity, howover, the feedback of signal components to the cathode l1 as lust described is undesirable from the stz-tndpoint that it accelerates further aging of -thc deficient screen areasV and thus nggravatcs the condition which gives risc to these undesirable amplitude variations of the video signal.

The operation effectcd by the. components 38-52 is very similar t0 ythat just described but with the difierence that no video signal is involved. The photo-multi plier tube 38 monitors the prevailing luminous intensity of the scanning spot created on the fluorescent screen 16 of the cathode ray tube l(l, and there is developed in an output circuit of the tube 3S an electrical ritenitor signal which varies in amplitude with variations of the scanning spot intensity. The very low frequencies components, from direct currentkto perhaps a or so cycles, of this nionitorrsignal are translated with substantial amplitude/by flic amplifier 39 to a control cirfrequency and midrange frequency amplitude variations tending to appear in the output monitor signal of the photo-multiplier tube 38. The units 39-42 thus provide for 'the tube 38 an amplitude control feedback loop similar in arrangement and operation to that provided for the video signal photo-multiplier tube.25. The concur-` rent control of the photo-multiplier tubes and 38 by their associated feedback loops has particular significance in connection with reduction of the phosphor noise contributed to the generated video signal by the inherent characteristics ofthe liuorescent screen 16.

y The uorescent screen of a cathode ray tube is con ventionally formed by settling phosphor particles out of t suspension in a liquid vehicle. After the screen has been settled and the liquid vehicle decanted, the screen is dried and the fabrication of the tube then progresses to completion, This screen formation process results in a layer of discrete phosphor particles adhering to the inner surface of the face plate of the tube. As a finely-focused electron beam scans the fiuorescent screen of the finished tube to create a luminous scanning spot', the particulate characteristic of the screen causes the luminous intensity of the scanning spot to vary within a small intensity range and at relatively high frequencies related to the range of phosphor particle sizes used in forming the fiuorescent screen. These intensity variations become more pronounced as the scanning spot is reduced in size to attain higher definition in exploring the image details of a scanned document, and are undesirable since they create a band of highfrequency noise in the video signal produced by the document scanning opera-tion.

As earlier explained, the feedback loopsassociated with the photo-multiplier tubes 25 and 3S operate to maintain substantially constant the peak amplitude of the video signal produced in the output circuit of the tube 25 and the peak amplitude of the monitor signal developed in the output circuit of the tube 38. In doing so, the amplitudes of phosphor noise components appearing in the video and monitor signals are also maintained substantially constant. It will be evident that any given 'instantaneous change of scanning spot intensity will produce corresponding concurrent amplitude changes of the video and monitor signals. Accordingly, when the video signal of the amplifier 26 and the high frequency phosphor noise" components of the monitoring signal translated by the amplifier 45 are concurrently supplied with opposed instantaneous polarities to the input circuit of the summing amplifier 46, the phosphor .noise components appearing in the video signal are reduced in amplitude or are efiectivelypcanceled by the amplifier 46 so that the video signal appearing in the output circuit 47 of the summing amplifier is essentially free of phosphor noise The amplifiers 26 and 39 have similar constructions and accordingly have equal phase shifts for the phosphor noise frequency components, and a high frequency amplifier 45 of conventional construction preferably should introduce a minimum of phase shift with respect the band of high frequency components which make up the phosphor noise This enables the opposite polarity phosphor noise frequency components of the video and monitor signals to cancel one another in the input circuit of the summing amplifier 46.

FIG. 2,is a circuit diagram representing the electrical circuit arrangement of the detectors 28 and 30, their associated respective amplifiers 29 and 31, the power supply unit 32, and the pltotoauuitiplicr tubc25. A similar circuit arrangement is employed for the photomultiplier tube 38 and its associated feedback loop components 39-42. The peak detector 28 `is shown in FIG. 2 as com* prised by a series diode rectifier device 52, a shunt connected condenser 53 with parallel connected resistor 54, and a series output-circuit isolating resistor 55. The time constant of the condenser 53 and resistor 54 is made sufficiently long by selection of the values of these components that only the lowest frequency components (including the direct current component) of the video signal are detected by the detector 28 and are supplied through the output circuit isolating resistor 55 to the linput circuit of the amplifier 29. The peak detector 30 similarly includes a series diode rectifier device 56, a shunt Aconnected condenser 57 and parallel connected resistor 58, and an output circuit series isolating resistor 59. The time constant of the condenser 57 and resistor 58 is selected sufriciently long that the detector 30 responds only to the midrange frequency amplitude variations of the video signal.

The power supply unit 32 is comprised by a self-oscillatory inverter 61 of conventional arrangement, and is energized through the output circuit of the amplifier 29 from a source of unidirectional energizing voltage E. The power supply includes power transistors 62 and 63 which alternately encrgizfe the center tapped primary Winding 64 of a saturablc core output transformer 65. The transistors 62 and 63 are rendcrcd alternately conductive by conventional feed-back windings 66 and 67 which energize the base electrodes of the respective transistors 62 and 63 through respective current-limiting resistors 68 and 69. The output transformer includes a plurality of secondary windings 70-74 which are connected to individual ones of a plurality of full-wave diode rectifier bridges 76-80 as shown. The output terminals of each rectifier bridge are connected to an individual one of a plurality of filter condensers 81-85 connected in series relation with one another between a ground terminal 86 and a high voltage output terminal 87. The filter condensers 81-84 are connected across individual ones of respective resistors 90-93 which are serially connected to provide a resistive potential divider, and the filter rcondenser 85 is connected across a plurality of resistors 94-99 which a-re serially connected as additional components of the resistive voltage divider last mentioned. The photo-multiplier tube 25 includes a photo emissive cathode which is energized from the junction of the voltage divider resistors 98 and 99, and includes a succession of electron multiplier electrodes lOl-104 which are energized as shown by individual potentials developed at the junction points of the voltage divider resistors 94- 98. The electron multiplier electrodes 10i-104 are those which first receive the stream of electrons emitted by the photo sensitive cathode 100 and comprising the generated video signal, and operate with relatively low electrode currents so that they may all be energized by that portion of .the resistive potential divider which is comprised by the resistors 94-98. The photo-multiplier tube 2-5 includes a succession of further electron multiplier electrodes 10S-109 which require larger operating currents by reason of their operation with an amplified electron stream, and accordingly these electrodes are energized as shown by current supplied directly to them from the filter condensers 81-84. The amplified video signal is devel'opedin an output circuit 110 of the highest current electron multiplier electrode 109 of the tube 25, and is supplied to the input circuit of the amplier 26 as shown.

The midrange amplifier 31 is shown in FIG. 2 as being comprised by a high gain preamplifier 114 and a power amplifier 115. The input of the pre-amplier 114 is coupled to the output circuit of thc peak detector 30, and the frequency pass band of this amplifier selects the mid frequency range of video signal components which are to be amplified and translated to the power amplifier 115.

The latter includes an output transformer 116 having a secondary winding 117 which is coupled through a coupling condenser 118 directly across the resistor 99 of the resistive potential divider which is comprised by the resistors 94-99.

In considering the operation of the FIG. 2 arrangement, it will be evident that the energization of the selfoscillatory inverter 61 is dependent in part upon the magnitude of the amplified output voltage developed in the' output circuit of the amplifier 29.- As the peak amplitude of the video signal voltage developed by the peak detector 28 increases, the output voltage of the amplifier 29 decreases to reduce the energization supplied to the inverter 61. This results in a reduction of the amplitudes of the alternating voltages developed in the secondary windings 70-74 of the inverter transformer 65, and reduces the magnitudes of the unidirectional energizing voltages applied to all of the electrodes of the photomultiplier tube 25. The multiplication ratio of the tube 25 is correspondingly reduced, and the peak amplitude of the video signal developed in the out-put circuit 110 of the tube 25 is smaller than it otherwise would be. Conversely, a reduced peak amplitude of the video signal voltage developed by the peak detector 28 increases the energization of the electrodes of the photo-multiplier tube 25. The resultant increased multiplication ratio of the latter increases the peak amplitude of the video signal developed in the output circuit 110. Thus any change of the peak amplitude of the video signal effects an inverse change of the multiplication ratio of the photomultiplier tube 25 by changed energization of all of the electrodes of this tube, and the resultant net change of video signal amplitude is correspondingly very small and becomes smaller as the gain of the feedback loop increases. y

The video signal voltage developed in the output circuit of the peak detector 30 is amplified by the amplifiers 114 and 115 and is applied through the output transformer 116 to change the instantaneous value of voltage developed across the resistive potential divider resistor 99 and thus the instantaneous unidirectional energizations of the photo cathode 100 and multiplier electrodes 101-105 of the tube 25. This is an alternating current change of energization yresulting from the midrange frequency amplitude changes of the video signal, and is such that an instantaneous increase of video signal amplitude eflects a decrease of unidirectional energization of the electrodes 100-105 to reduce the multiplication ratio (and thus the gain) effected by the tube 25. Conversely, an instantaneousireduction of video signal amplitude at the detector 30 increases the multiplication ratio 0f the tube 25. It will be noted that this alternating current energization control is accomplished at the low-current-level electrodes 100-105 of the photo-multiplier tube 25 where the control may be more readily effected. This low level control of the multiplication ratio of the tube 25 is satisfactory, however, since the amplitudes of the higher frequency amplitude variations tending to appear in the video signal at the detector 30 are usually much smaller than those at the detector 28. Here again, the resulting net higher frequency amplitude changes of the video signal are reduced with increasing values of gain of the midrange feedback loop.

As earlier mentioned, an arrangement similar to FIG. 2 is used with respect the FIG. 1 photo-multiplier tube 38 and associated feedback loop units 39-42 to maintain substantially constant the peak amplitude of the monitor signal developed in the output circuit of the tube 38.

It will be apparent from the foregoing description of the invention that a video signal generator embodying the invention is one particularly suitable for document character recognition applications and compensates for many operational conditions which tend undesirably to change the peak amplitude of the generated video signal. A video signal generator embodying the invention not only compensates for video signal peak amplitude changes occurring at relatively low frequencies, but also enables compensation for high-frequency amplitude variations occasioned by phosphor noise" prevalent in flying spot scanners.

While a specific form of the invention has been described for purposes of illustration, it is contemplated that possible changes may be made without departing from the spirit of the invention.

What is claimed is:

1. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a scan raster to explore incremental-area reflection brightness contrasts of said image, photo-multiplier means responsive to light energy reflected from said image and modulated in intensity by the light-spot exploratory scanning thereof for generating an electrical video signal having an instantaneous amplitude at least in part proportional to the incremental-area image reflective brightness and to the prevailing peak intensity of said light spot, means for deriving a first electrical control signal having an amplitude proportional to the average prevailing peak intensity of said light spot and a second electrical control signal having an amplitude proportional to the prevailing image reflective peak brightness during scanning bysaid light spot, and means responsive to said control signals for varying inversely with changes in the amplitude of each thereof the multiplication ratio of said photo-multiplier means to reduce variations in the peak amplitude of said generated video signal resulting from variations in the average peak intensity of said light spot and the reflective peak brightness of said image.

2. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a scan raster to explore incremental-area reflection brightness contrasts of said image, a source of electrical energization, a photo-multiplier tube energized'by said source and responsive to light energy reflected from said image and modulated in intensity by the light-spot exploratory scanning thereof for generating an electrical video signal having an instantaneous -amplitude at least in part proportional to the incremental-area image reflective brightness and to the prevailing peak intensity of said light spot, means for deriving a first electrical control signal having an amplitude proportional to the average prevailing peak intensity of said vlight spot and a second electrical control signal having an amplitude proportional to the prevailing image reflective peak brightness during scanning by said light spot, and means responsive to said contrlo signals for controlling said energizing source to vary inversely with changes in the amplitude of each thereof the multiplication ratio of said photo-multiplier tube to reduce variations in the peak amplitude of said generated video signal resulting from variations in the average peak intensity of said light spot and the reflective peak brightness of said image.

3. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a scan raster to explore incremental-area reflection brightness contrasts of said image, a source of electrical energization, a photo-multiplier tube energized by said source and responsive to light energy reflected from said image and modulated in intensity by the light-spot exploratory scanning thereof for generating an electrical video signal having an instantaneous amplitude at least in part proportional to the incrementalarea image reflective brightness and to the prevailing peak intensity of said light spot, means for deriving a rst electrical control signal having an amplitude proportional to the average prevailing peak intensity of said light spot and a second electrical control signal having an amplitude proportional to the prevailing image reflective peak brightness during scanning by said light spot, means responsive to said rst control signal for controlling said energizing source to vary inversely with relatively slow changes in the amplitude of said first control signal the multiplication ratio of said photo-multiplier tube, and means responsive to said second control signal for additionally controlling said energizing source to vary inversely with relatively rapid changes in the amplitude of said second control V signal the multiplication ratio of said photo-multiplier tube, thereby to reduce variations inthe peak amplitude of said generated video signal resulting from variations in the average peak intensity of said light spot and the reflective peak brightness of said image.

4. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a sean raster to explore incremental-area reflection brightness contrasts of said image, a source of electrical energization, a photomultiplier tube energized by said source and responsive to light energy reflected from said image for generating an electrical video signal, means having a low-pass fre- `queney-band translation ycharacteristic and 2 responsive to` relativelyslow ehangesin the peak amplitude; of said` video signal for controlling said `energization source to vary the multiplication-ratio of said tube inversely with said slow changes of said video signal peak amplitude, and means having a high-pass frequency-band translation characteristic and responsive to more rapid changes in the peak amplitude of said video signal for controlling said energizing source to vary the multiplication ratio of said tube inversely with said rapid changes of said video signal peak amplitude.

5. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a scan raster to explore incremental-area reflection brightness contrasts of said image, a `source of electrical energization, a photomultiplier tube having plural electron multiplier electrodes energized by `saidsource and having a4 photoemissive electrode responsive to light energy'reflected Afrom said image for generating an electrical video signal,

eans for amplifying said video signal, means having a low-pass frequency-band translation characteristic and responsive to relatively slow changes in the peak amplitude of said amplified video signal for controlling the energization supplied by said energization source to all of said electron-multiplier electrodes to vary the multiplication ratio of said` tube inversely with said slow changes of said video signal` peak amplitude, and means having a high-pass frequency-band translation characteristic and responsive to more rapid changes in the peak `amplitude of said amplified video signal for controlling the energization supplied by said energizing source to plural low-level ones of said electron-multiplier electrodes to vary the multiplication ratio of said tubeinversely with said `rapid changes of said video signal peak amplitude.

.6. A video signal generator comprising means for creating and projecting onto an image to be transmitted a moving spot of light tracing a sean raster to explore incremental-area reflection brightness contrasts of said image, a source of electrical energization, a photomultiplier tube energized by said source and responsive to light energy reflected from said image for generating an electrical video signal, a video peak detector and lowpass video amplifier `responsive to relatively slow changes in the peak amplitude of said video signal for controlling said energizatien source to vary the multiplication ratio of said tubeinversely with said slow changes of said video p signal peak amplitude, and a video peakdetector and high-pass video amplifier responsive to more rapid changes in the peak amplitude of said video signal for controlling said energizing source to vary the multiplication ratio of said tubeinversely with said rapid changes of said video signal peak amplitude.

7. A video signal generator comprising means for creating and projecting onto an image to be transmitted a vfor generating an electrical video signal; a video peak detector and low-pass video amplifier responsive to relatively slow changes in the peak amplitude of said video signal for controlling said energization source to vary concurrently the magnitudes of energizations of all of said plural electrodes, and thus the multiplication ratio of said tube, inversely with said slow changes of said video signal peak amplitude; and a video peak detector and high-pass video amplifier responsive tomore rapid changes in the peak amplitude of said video signal-for controlling said energizing source to vary concurrently the energizations of a lower-voltage group ofsaid electrodes, and thus the multiplication ratio ofsaid tube, inversely with said rapid changes of said video signal peak amplitude. 8. A video signal generator comprising means including a cathode-ray tube for creating.` and projecting onto an image to be transmitted a moving spot of light tracing a scan raster to explore incremental-area reflection brightness contrasts of said image, a source of electrical energization, a photo-multiplier tube energized by said source and responsive to light energy reflected from said image for generating an electrical video signal, means having a low-pass frequency-band translation characteristic and responsive to relatively slow changes in the peak amplitude of said video signal for controlling said energization source to vary the multiplication ratio of said tube inversely with said slow changes of said video signal peak amplitude, and means having a high-pass frequency-band translation characteristic and responsive to more rapid changes in the peak amplitude of said video signal for controlling inversely therewith both the cathode-ray beam ycurrent of?` said cathode-ray tube and through said ensaid first energizing source the multiplication ratio of said first tube, a second photo-multiplier tube and a seeond source of energization therefor to derive an electrical control signal having an amplitude varying both with the average peak brightness and fluctuating brightness of said light spot during a complete scan raster, means responsive to peak-brightness amplitude changes of said control signal for controlling inversely therewith and through said second energizing source the multiplication ratio of said second tube, and means for combining with opposite polarities the fluctuation-brightness amplitude changes of said control signal and said video signal to derive an output video signal having reduced amplitude changes caused by fluctuating brightness of said light spot.

References Cited by the Examiner UN iTED STATES PATENTS 9/1961 Bailey 178-7.2 2/1965 Evans 1787.1 

6. A VIDEO SIGNAL GENERATOR COMPRISING MEANS FOR CREATING AND PROJECTING ONTO AN IMAGE TO BE TRANSMITTED A MOVIGN SPOT OF LIGHT TRACING A SCAN RASTER TO EXPLORE INCREMENTAL-AREA REFLECTION BRIGHTNESS CONTRASTS OF SAID IMAGE, A SOURCE OF ELECTRICAL ENERGIZATION, A PHOTOMULTIPLIER TUBE ENERGIZED BY SAID SOURCE AND RESPONSIVE TO LIGHT ENERGY REFLECTED FROM SAID IMAGE FOR GENERATING AN ELECTRICAL VIDEO SIGNAL, A VIDEO PEAK DETECTOR AND LOWPASS VIDEO AMPLIFIER RESPONSIVE TO RELATIVELY SLOW CHANGES IN THE PEAK AMPLITUDE OF SAID VIDEO SIGNAL FOR CONTROLLING SAID ENERGIZATION SOURCE TO VARY THE MULTIPLICATION RATIO OF SAID TUBE INVERSELY WITH SAID SLOW CHANGES OF SAID VIDEO SIGNAL PEAK AMPLITUDE, AND A VIDEO PEAK DETECTOR AND HIGH-PASS VIDEO AMPLIFIER RESPONSIVE TO MORE RAPID CHANGES IN THE PEAK AMPLITUDE OF SAID VIDEO SIGNAL FOR CONTROLLING SAID ENERGIZING SOURCE TO VARY THE MULITPLICATION RATIO OF SAID TUBE INVERSELY WITH SAID RAPID CHANGES OF SAID VIDEO SIGNAL PEAK AMPLITUDE. 